disco lug landing [RE-wrenches]

Matt Lafferty mlafferty at universalenergies.com
Tue Jun 14 07:37:47 PDT 2005


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Wrenches,

Marco M wrote>

> I've got a non-residential PV application where the only reasonable
> utility tie-in location is the existing 200A fused main service
disconnect.
>  I'm going to use a 100A AC fused disconnect which will carry the
> combined output of several inverters.  The question is: where to land
> the output of the 100A inverters' disco--on the line or load side of the
> existing 200A main service disco?

2002 NEC 690.64(B)(2) stipulates that:  The sum of the ampere ratings of
overcurrent devices in circuits supplying power to a busbar or conductor
shall not exceed the rating of the busbar or conductor.

There is an Exception, which states:  Exception: For a dwelling unit, the
sum of the ampere ratings of the overcurrent devices shall not exceed 120
percent of the rating of the busbar or conductor.

In Marco's case, this is NOT a residential (dwelling) application, so the
"120% rule" does not apply.  Assuming that the busbar is rated at 200 Amps
and the Main Breaker/Fuse is also rated at 200 Amps, the only "legal" place
to land the interconnection is on the line side of the Main Disconnect.  Be
sure to follow the tap rules (including conductor length limits) and
schedule a power outage for the interconnection.

This is basically "standard practice" for non-residential systems since most
Service/Distribution Panels are fed via devices rated at or very near the
busbar ratings.  One fairly common exception in small 3-Phase Services is a
225 Amp busbar with a 175 or 200 Amp Main.  (Leaving room for 50 or 25 Amps
of generation respectively).  

Here in NorCal we are seeing more Building Departments checking for this on
both residential and non-residential systems.  One county in particular
(Placer) has even gotten themselves a red stamp that they apply to permit
drawings... "Thou shalt not exceed the busbar rating of the interconnection
panel"....

Peace and Palms! 

Matt Lafferty
mlafferty at universalenergies.com
(415) 252-0343
(415) 748-6010 Cell
(916) 914-2247 Fax

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